1. Field of the Invention
The present invention relates to a socket for an electrical part for detachably accommodating electrical parts such as semiconductor devices (called “IC package” hereinafter), to establish an electrical connection between the electrical part and a wiring board.
2. Prior Art of the Invention
In conventional art, an IC socket has been known as a “socket for an electrical part” of this kind for detachably accommodating, for example, an IC package as an “electrical part.” And one of the examples of this kind of the IC socket is shown in FIGS. 7 to 10.
The conventional IC socket 1 has a socket body 3 to be mounted on a wiring board 2, and a contact unit 5 which is fixed to the socket body 3 and is provided with a plurality of contact pins 4.
The contact unit 5 has a unit body 7 fixed to the socket body 3 by an attachment member 8, and the contact pins 4 are provided to the unit body 7.
The contact pin 4 (details are not shown) has an upper contact portion 4a designed to contact with a solder ball 6b as a terminal of the IC package 6 accommodated on the unit body 3 and a lower contact portion 4b designed to contact with an electrode portion 2a of a wiring board 2. Between both contact portions 4a, 4b, a spring member (not shown) that urges both contact portions 4a and 4b so as to move them away from each other is arranged.
A contact pressure between the upper contact portion 4a and the solder ball 6b is designed to be equal to a contact pressure between the lower contact portion 4b and the electrode portion 2a of the wiring board.
In such IC socket 1, as shown in FIGS. 7 and 8 in which the socket body 3 is attached to the wiring board 2 and secured by a screw 10, the IC package 6 is first accommodated on the unit body 7, a cover member 9 pushes the IC package 6 as seen from the state shown in FIG. 7 to the state shown in FIG. 8, so that the contact pin 4 is compressed, and then the downward displacement of the IC package 6 is stopped when the IC package 6 abuts on the unit body 7.
At this time the contact pin 4 is compressed against the urging force of the spring member, and the reaction force in response to the amount of the compression acts on both contact portions 4a and 4b, thereby establishing electrical contacts with equal contact pressure on both the contacts between the upper contact portion 4a and the solder ball 6b of the IC package 6 and between the lower contact portion 4b and the electrode portion 2a of the wiring board 2.
The socket of this kind is disclosed in, for example, WO 01/037381.
In such conventional socket, however, a gap or distance C1 as shown in FIGS. 7 and 8 is set up, in consideration of a difference or dispersion in the thickness of the unit body 7, between the unit body 7 and the wiring board 2 in the case where the unit body 7 is fixed to the socket body 3, or a gap or distance C2 as shown in FIGS. 9 and 10 is set up between the socket body 3 and the wiring board 2 at the stage where the unit body 7 abuts on the wiring board 2.
In the former case shown in FIGS. 7 and 8, even when the contact pin 4 is compressed to the maximum limit, the contact pin 4 can not be compressed beyond the distance of C1. Therefore, the amount of compression of the contact pin 4 becomes small, causing decrease in contact pressure of the contact portion 4a or 4b with respect to the solder ball 6b of the IC package 6 or the electrode portion 2a of the wiring board 2.
On the other hand, in the latter case shown in FIGS. 9 and 10, when the contact pin 4 is compressed to the maximum limit, the contact pressure of the contact portion 4a or 4b with respect to the solder ball 6b of the IC package 6 or the electrode portion 2a of the wiring board 2 can be increased more than the case of FIGS. 7 and 8. However, warping could be occurred in the socket body 3 or the wiring board 2 when the socket body 3 is fixed to the wiring board 2 by the screw 10 because of the presence of the distance C2 between the socket body 3 and the wiring board 2.